A common process sequence in manufacturing integrated circuits and other electronic devices is to deposit a dielectric layer over a semiconductor or metal region on a substrate, then etch a number of openings in the dielectric so that each opening exposes a contact area on the semiconductor or metal region, then fill each opening with an electrically conductive material so as to form a plug that makes electrical contact with the contact area.
It is very difficult to fill an opening having a very narrow width or a high aspect ratio, that is, a high ratio of height to width. In such an opening, the metal or other electrically conductive material deposited to fill the opening can agglomerate while the opening is being filled, thereby preventing the metal from flowing into and filling the portion of the opening below the agglomeration. The resulting void renders the plug defective.
Conventional processes for forming a plug typically attempt to prevent the formation of voids by depositing a wetting layer or adhesion layer, typically composed of titanium nitride, on the side wall of an opening before depositing the metal used to fill the plug. However, we have found that a titanium nitride wetting layer can be inadequate to prevent the formation of voids in openings having a high aspect ratio, for example, an aspect ratio of 6 or more. This is especially true of titanium nitride layers formed by CVD using a metallo-organic precursor gas, which is a preferred method of depositing titanium nitride when the substrate temperature must remain low.
Even if a narrow opening is completely filled without voids, the plug may have undesirably high resistivity because the crystallographic structure of the conductive material of the plug may include a large number of small grains rather than a small number of large grains. A large number of grain boundaries within the plug increases its electrical resistance.